Aluminum and aluminum alloys are desirable materials for component fabrication in many industries, such as aircraft and automotive industries, because they are strong, relatively inexpensive, lightweight, and generally resistant to corrosion. The corrosion resistance of aluminum and aluminum alloys results from self-renewing passive oxide films that develop on their surfaces and advantageously create a barrier against corrosion. While effective, the passive oxide films may break down locally causing small areas of the aluminum alloy to become exposed for localized corrosion in the form of pitting corrosion, intergranular attack, crevice corrosion, and stress cracking corrosion. Such localized corrosion may occur, for example, when the exposed areas of the aluminum or aluminum alloy come into contact with corrosive agents in the environment such as water, air, and chlorides. The susceptibility of aluminum and aluminum alloys to localized corrosion may limit the use of these materials, for example, in fan blades and compressor airfoils in gas turbine engines of aircraft, which frequently operate near coastal environments containing high levels of corrosive chlorides from wind-blown seawater mist. As such, fan blades in gas turbine engines are frequently made from heavier and more expensive metals such as titanium.
The development of strategies designed to protect aluminum and aluminum alloy based structures from localized corrosion may be advantageous for promoting the use of these materials in a variety of industries. One strategy may be to apply paint coatings, such as chromated epoxy based primers, to the surfaces of aluminum alloy fan blades and compressor airfoils to form a barrier that shields the surfaces from coming into direct contact with corrosive agents in the environment. However, if introduced into gas turbine fan blades and compressor airfoils, such coatings may be short lived as they may be subject to erosion and/or damage upon impact with foreign objects such as birds, ice, and sand during fan operation.
One known approach for protecting steel structures from corrosion involves introducing a sacrificial anode in electrical contact (such as metal to metal contact) with a portion of a surface of the steel structure. This strategy has been described, for example, in U.S. Pat. No. 3,870,615 and has been successfully applied for the protection of steel hulls of ships, offshore oil rigs, as well as pipelines which would otherwise be exposed to the corrosive effects of seawater or groundwater. The sacrificial anodes may protect such steel structures from corrosion by a cathodic protection mechanism in which the sacrificial anode is preferentially corroded over the steel structure upon exposure to corrosive conditions in the environment. In this regard, the sacrificial anode may be formed from a metal such as zinc, magnesium, or aluminum, which have lower electrochemical potentials that the iron material forming the steel, in order to provide a driving force for the sacrificial anode to preferentially corrode. In the presence of water and electrolyte, the sacrificial anode and steel structure create a galvanic cell, with oxidation (corrosion) occurring at the sacrificial anode and the released electrons being shuttled to the steel structure. Once the sacrificial anode has been substantially consumed by corrosion, a new sacrificial anode layer may be applied to the surface of the steel structure to further extend protection.
The development of similar cathodic protection strategies for aluminum and aluminum alloy based structures are still wanting, however. Clearly, a system is needed to protect aluminum and aluminum alloy structures from localized corrosion in order to reduce or minimize the risks of structural failure.